7.2.3.14 $DUMPDEF: Dump macro definitions to the terminal

In the call ‘$DUMPDEF(m1, m2, ...)’, m1, m2, and so on are macro calls (with arguments if appropriate). Two lines of output are generated for each argument. Line 1 is the macro definition; line 2 is its expansion using the provided arguments.

One can use this built-in to debug one’s own macros, or to find out the secrets of FWEB’s built-ins. As an example, if one says

$DUMPDEF($EVAL(2^^4))@%

it responds with the two lines

$EVAL($0) = $$EVAL($0)
$EVAL(2**4) = 16

(The $n notation indicates the n-th argument of the macro.) If one replaces $EVAL with $$EVAL in the above $DUMPDEF, it will respond

$$EVAL($0) = <built-in>
$$EVAL(2**4) = 16

The purpose of code such as ‘$EVAL($0) = $$EVAL($0)’ is to ensure that the argument of $EVAL is expanded if it contains macros; the primitive function $$EVAL does not do that expansion automatically.

Names indicated as ‘<built-in>’ by $DUMPDEF may be redefined as ordinary macros, but this is in general a very bad idea; other parts of FWEB may mysteriously stop working.